FIG. 9 shows a conventional image forming apparatus. This image forming apparatus includes a main body 1, a paper tray unit 2 having a paper tray and a paper feed roller 3 for feeding sheets of paper by ones into the main body 1 from the paper tray, a skew-correcting device 4 constituted by a pair of rollers which transports the paper (represented by the numeral 8 in FIG. 9) fed by the paper feed roller 3 while correcting the skew of the paper, a paper sensor 5 for detecting the front edge of the paper, an image forming unit 6 having an image drum and a transference roller for forming an image on the paper, and a fixing unit 7 having a heater and rollers for fixing the image on the paper.
When the paper supplied from the paper tray by the paper feed roller 3 has reached the skew-correcting device 4, the paper sensor 5 detects the front edge of the paper and generates a signal. The paper feed roller 3 continues to rotate even after receiving the signal generated by the paper sensor 5 to further feed the paper by a predetermined amount. In consequence, the skew of the paper, that is, the inclination of the paper with respect to the direction along which the paper is transported, is corrected, since the front edge of the paper strikes on the nipping portion of the skew-correcting device 4 which is at a standstill, and the paper therefore bends there. After that, the rollers of the skew-correcting device 4 start to rotate for transporting the paper to the image forming unit 6. The image formed on the paper by the image forming unit 6 is fixed by the fixing unit 7. After that, the paper is discharged.
It is known to provide such an image forming apparatus with a plurality of paper tray units to enable users to set a large number of sheets of paper as disclosed in Japanese Patent Application Laid-Open No. 11-193149. Each paper tray unit has a skew-correcting device. The skew of the paper supplied from one paper tray unit is corrected by the skew-correcting device of this one paper tray unit and also by the skew-correcting devices of other paper tray units disposed above this one paper tray unit. To be more specific, in a case where the paper is supplied from one of the paper tray units (here, referred to as “paper tray unit in question”), a paper feed roller installed in the paper tray unit in question continues to rotate even after a paper sensor installed in the paper tray unit in question detects that the paper has reached a skew-correcting device installed in the paper tray unit in question in order to further feed the paper by a predetermined amount, whereby the skew of the paper is corrected. Even when a paper sensor installed in another paper tray unit disposed immediately above the paper tray unit in question detects that the paper has reached a skew-correcting device of this paper tray unit, the paper feed roller and the rollers of the skew-correcting device of the paper tray unit in question continue to rotate to feed the paper by a predetermined amount for correcting the skew of the paper again. Thus, the paper supplied from the paper tray unit in question goes into the main body after undergoing the skew correction multiple times by the skew-correcting devices of the paper tray unit in question and other paper tray units disposed above the paper tray unit in question.
As previously explained, when the paper reaches the skew-correcting device disposed upstream from the image forming unit, the skew of the paper is corrected since the paper strikes on the skew-correcting device and is bent there. After the skew of the paper is corrected, the rollers of the skew-correcting device start to rotate to transport the paper to the image forming unit where an image is formed on the paper, and after that the paper is discharged.
Conventionally, in the case of using a plurality of the paper tray units having the structure described above for supplying paper to the image forming apparatus, the paper supplied from one of the paper tray units stops and undergoes the skew correction at each of the skew-correcting devices of other paper tray units through which it passes. Accordingly, there is a problem that printing speed is lowered since the paper stops again and again before it goes into the main body of the image forming apparatus, especially in supplying paper from tray units distant from the main body of the image forming apparatus. There is also another problem that a complicated control mechanism is required for controlling execution of the skew correction by each of the paper tray units.